The present invention relates generally to a watchcase glass and more particularly to a technique for securing the glass to the watchcase to obtain a waterproof construction.
A glass of a watchcase has conventionally been secured to the case by adhesives. In recent years, a construction in which a glass is secured to a case by placing a plastic packing at a peripheral portion of the case and forcing the glass in the case with an interference fit has been mainly used for obtaining a waterproof watchcase.
FIG. 1A shows a sectional view of a conventional watchcase formed by the adhering method and FIG. 1B shows the shape of the glass used in the conventional watchcase. A glass 1 is adhered to a case 2 by adhering a glass-receiving surface 2S of a peripheral case portion 2A and an outer peripheral bottom surface 1S of the glass by adhesives. This method has been generally and widely used because of its extreme simplicity. This method, however, has rarely been used in connection with waterproof watches and has been used essentially only for non-waterproof watchcases because of the lack of strength of the adhesives, deterioration of the adhering force with elapsed time, and the inferiority in appearance due to the relatively large adhering area needed if used for a waterproof watchcase. Moreover, although the watchcase made by the adhering method is simple in construction, the manufacturing cost is not so inexpensive in view of the number of man-hours needed to take appropriate countermeasures to compensate for the useful life of the adhesives, temperature control, and for the swelling of the adhesives.
On the other hand, FIG. 2A shows a conventional watchcase formed by the force-fit method in which the glass 1 is forced in the case 2 using a plastic packing 3 and this method has recently been widely used for waterproof watches. FIG. 2B shows the shape of the glass 1 used in the conventional watchcase of FIG. 2A. This method has come into wide use because the plastic packing deteriorates less with elapsed time than adhesives and is easier to handle during the course of manufacture. However, the reduction in size and thickness of the watchcase is limited, as shown in FIG. 2C, because some width w and height h of the packing 3 are needed according to the molding conditions of the plastic packing and the arrangement of the plastic packing in the case. The width w of the packing should be 0.3-0.4 mm before assembly and 0.2-0.25 mm after assembly, and the height h should have a maximum value less than three times the value of w. If the height h is more than three times the width w, the assembling of the packing 3 becomes extremely difficult because the packing 3 may be flexed or pulled in the peripheral case portion 2A when the glass 1 is forced in the case 2.